Weird node setup when importing .obj file
That's just how the importer interprets materials that come with obj format. If there was some description of a material in the .mtl file it would construct this shader according to it.

You can replace them easily if you delete the material and make a new one. If for some reason that was not acceptable you could use Python to replace the nodes. Let's say it would be convenient for you to have standard Material Output node and a Diffuse BSDF and Glossy BSDF mix by Fresnel connected to it and you wanted to replace the nodes only for the materials that are applied to the currently selected objects. Python script would go like that:

import bpy

done_materials=[]

for every_object in bpy.context.selected_objects:
    try:
        for every_slot in every_object.material_slots:
            if every_slot.material not in done_materials:
                #print(every_slot.material.name)
                nodes = every_slot.material.node_tree.nodes
                links = every_slot.material.node_tree.links
                for every_node in nodes:
                    nodes.remove(every_node)
                diff = nodes.new('ShaderNodeBsdfDiffuse')   
                diff.location = (-190,100)
                gloss = nodes.new('ShaderNodeBsdfGlossy')   
                gloss.location = (-190,-20)
                mix = nodes.new('ShaderNodeMixShader')   
                mix.location = (10,95)
                Fres = nodes.new('ShaderNodeFresnel')   
                Fres.location = (-190,200)
                Fres.inputs[0].default_value=1.3
                out = nodes.new('ShaderNodeOutputMaterial')   
                out.location = (200,95)
                links.new(mix.outputs[0], out.inputs[0])
                links.new(Fres.outputs[0], mix.inputs[0])
                links.new(diff.outputs[0], mix.inputs[1])
                links.new(gloss.outputs[0], mix.inputs[2])
                done_materials.append(every_slot.material)
    except: 
        pass


And of course you would just need to have that pasted into a new text block in the Text Editor, select all the objects that you want processed and hit Run Script.